The present invention relates to a screw compressor that includes two mutually coacting helical screw rotors housed in a rotor housing. The housing includes first and second end walls that are interconnected by a barrel wall. The housing has an internal configuration that corresponds to two mutually intersecting cylinders and the first end wall includes an inlet opening while the barrel wall includes an outlet opening adjacent the second end wall and further includes an opening which extends from the outlet opening towards the first end wall and widens over both cylinder surfaces. The housing also includes a movable part which coacts with said further opening and which in a first position engages the housing such as to form the mutually intersecting cylindrical surfaces and which in a second position is out of engagement with the housing and therewith spaced from said further opening. The compressor includes a maneuvering device for selective positioning of the movable part either in its first or in its second position.
With respect to compressors of this kind it is necessary in many applications to supply compressed working fluid intermittently. In order to avoid the use of complicated, and therewith expensive solutions in respect of these applications, for instance the use of a coupling device or clutch mechanism between the compressor and its drive means, or to avoid the activation and deactivation of said drive means, compressors of this kind are normally provided with means which allow the compressor to idle during those periods when compressed fluid is not required. This is usually achieved by opening a free connection between the high pressure side of the compressor and its low pressure side for return of the working fluid.
Such arrangements are described, for instance, in U.S. Pat. Specification Nos. 3, 527, 548, 3, 759, 636, 4, 119, 392, 4, 799, 865 and 4, 993, 923.
The compressor thus taught by U.S. Pat. No. 3, 527, 548 has hollow axles or shafts whose interiors communicate with the outlet through one opening, and with the inlet.
This opening can be closed by a pressure actuated valve against the force of a spring that functions to hold the valve open.
The compressor taught by U.S. Pat. No. 3, 759, 636 includes in its high pressure end section a valve which can be opened against an oil pressure through the medium of the outfeed pressure together with a pull spring, said oil pressure acting in the direction opposite to said outfeed pressure. The valve opens a connection between the compressor outlet and compressor inlet.
The compressor taught by U.S. Pat. No. 4, 119, 392 teaches a female rotor which is shorter than the distance between the end walls and which can be moved axially by a piston that either seals against the high pressure end-wall at full load or leaves a gap to the high pressure end-wall so as to bring the high pressure side into fluid connection with the inlet via the rotor grooves so as to provide an idling facility.
U.S. Pat. No. 4, 799, 865 teaches a compressor that includes an axially movable endwall at the high pressure end. The end wall is held closed by springs and can be lifted by a piston, or plunger, so as to relieve the load on the compressor.
Finally, U.S. Pat. No. 4, 993, 923 teaches a pressure actuated slide valve that is disposed around the high pressure end of each rotor shaft, said valve providing fluid connection between outlet and inlet when open.
A compressor that includes two mutually coacting screw rotors and used as an engine supercharger is known from U.S. Pat. Specification U.S. Pat. No. 4, 744, 734. Arranged at the end of the outlet passageway of this compressor is a disc-shaped throttle valve. The throttle disc is fixed in a shaft that can rotate about its long axis. The compressor also includes a bypass passageway which has connection with the compressor inlet. Two openings in the housing connects the by-pass with the rotor housing over one of the rotors, and throttle valves pivotally mounted in said passageway function to close and open said connection. Resilient arms fastened in the throttle valve shaft cause the pivotal throttle valves to either close or open the two openings in response to corresponding rotation of the shaft. When the two openings are closed in response to rotation of said shaft, the outlet passageway is opened by the valve disc, and vice versa.
The European Patent Application EP-A2-0 484 885 teaches a screw rotor compressor having an inlet at one end and a movable piston in the other axial end of the compressor housing, adjacent the outlet. The piston moves generally perpendicularly to the axes of the screw rotors. Maximum compression is obtained, when the piston is situated closest to the rotors. The size of the outlet opening increases as the piston is moved away from this position, so as to obtain lower compression.
One drawback with this latter compressor is that it cannot be made sufficiently compact to suit many purposes. The movable piston by means of which the degree of compression obtained with the compressor is adjusted must have a significant length in its axial direction. It must be possible to move the piston towards and away from a position of abutment in relation to the rotors without sticking or jamming. The forces acting on the piston from the working chamber of the compressor vary and are greatest adjacent 5 the outlet.
Another drawback is that the piston may not rotate about its own axis when moved axially in order to change the compression ratio of the compressor. Because the piston is situated above both rotors, any such rotation would cause the piston to come into contact with the rotors and therewith damage the same.
One object of the present invention is to eliminate the drawbacks of the solution described in the European Patent Application with respect to changing the compression ratio in a compressor.
Another object of the invention is to provide a novel and improved method of short-circuiting the working fluid with respect to idling of a compressor.
Still another object of the invention is to provide a simple reliable compressor where the requirements on valve tolerances are low with respect to adjusting the compression ratio.
These objects have been achieved in accordance with the present invention by means of a compressor that includes a modification in which an operating device or maneuvering device functions to actuate a rotatable shaft which is mounted on the compressor or firmly fixed relative thereto and which is firmly connected to a movable part of the compressor barrel through the medium of a connecting element.
Because the valve plate, i.e. the movable part of the barrel wall, is rotatable about an axis, it can be readily rotated out of engagement with the barrel wall and distanced from the rotors. This rotation opens the closed working chambers of the compressor, therewith immediately relieving the compressor of load. The production of compressed fluid is changed immediately, either in one step or ceases almost entirely. When the movable part extends from the outlet opening to the opposite end wall, or preferably terminates a short distance from said wall, all working chambers will come immediately into fluid connection with each other when the movable part is lifted or distanced from abutment with the stationary part of the barrel wall. Losses due to backflow or reflux are reduced to a minimum. Consequently, the movable part need only move to a very small extent in order to provide idling conditions. Only one simple maneuvering device is required to obtain this movement, for instance a motor or a spring which acts on the shaft in some appropriate manner. In one embodiment of the present invention, the opening in which the valve plate is disposed terminates short of the first end wall so that a certain degree of compression will be obtained even when the valve cover is not in engagement with the barrel housing.
Another important advantage afforded by the present invention is that the movable part can be fixed through the medium of a connecting element in said shaft. Because the connecting element has a broad and long abutment area with said shaft, the movable part will be positioned exactly in its intended place in the compressor housing in engagement with the barrel housing, and will lie in the vicinity of the rotors with only a small amount of clearance therebetween and in the absence of further guide elements.
Attachment of the connecting element on the movable part and in the shaft including bores whose diameters are larger than the diameters of the screws or other fasteners used, the shaft may be positioned at a wrong angle without any negative effect on the valve function.
Although the movable part can be arranged anywhere in the barrel wall, the placement of said movable part perpendicular to the intersection plane of the cylinders is preferred, since this provides a through-passing connection with the grooves of both rotors.
The compressor housing is made of a metallic material, for example aluminum. The movable part, however, can be made of some other material, for example a polymeric material.
Preferred embodiments of the invention are made apparent in the depending claims.